The Advanced Range Telemetry (ARTM) program is a United States Department of Defense tri-service telemetry modernization project whose goal is to assure that all testing and training ranges are able to use telemetry as necessary to carry out their respective missions. Multi-h Continuous Phase Modulation (CPM) has been selected by the ARTM Joint Programs Office as the Tier II ARTM waveform, because it offers significant improvements over both legacy telemetry waveforms such as pulse coded modulation/frequency modulation (“PCM/FM”) and the previous Tier I waveform known as the Feher-patented quadrature-phase-shift keying (“FQPSK”) in terms of spectral containment and detection efficiency, while retaining a constant envelope characteristic.
The ARTM Tier II modulation format is a multi-h continuous phase modulation. Those skilled in the art will appreciate that the multi-h continuous phase modulation format has a constant envelope and narrow bandwidth. Current implementations of receivers for multi-h continuous phase modulation experience several difficulties, including that the branch metrics are solely a function of the data in the multi-symbol observation window. That is, the influence of previous observations is not passed along in the form of a cumulative path metric. The skilled artisan will appreciate that the performance improves as the multi-symbol observation length increases; however, the penalty for this is that trellis complexity increases exponentially with increasing observation length. In addition, the current implementations perform poorly for practical multi-symbol observation lengths with respect to the Advanced Range Telemetry Tier II modulation format. Thus, the existing optimal maximum likelihood sequence estimation receiver for continuous phase modulation may have high complexity, both in trellis size and coherent demodulation requirements.
Lock detectors have been provided for use with digitally modulated signal detectors. Previously known carrier lock detectors are typically implemented as phase lock loop detectors, delay-locked loop detectors, or signal power detectors. The performance of lock detectors of these types is dependant on the carrier signal amplitude. This being the case, at low levels of signal to noise ratio, previously known lock detectors may indicate an out-of-lock condition even though the carrier is still locked.
Also known in the art, are more advanced lock detectors that are signal amplitude independent. However lock detectors of this type exhibit high complexity.
Thus, there is a need for a carrier lock detector and method for carrier lock detection that overcome the above-mentioned problems.
It would be desirable to have a carrier lock detector and method that are not dependant on the amplitude of the carrier signal.
Further, it is desirable to be able to reliably discriminate between a digitally modulated informative signal with a very poor signal to noise ratio, and no signal whatsoever.
It would be also desirable to have a carrier lock detector and method capable of providing reliable lock information even at very low signal to noise ratio, while simplifying hardware requirements.
In view of the above, there is provided in accordance with the embodiments disclosed herein an improved, noncoherent receiver capable of providing multi-symbol observation. Further in view of the above, there is provided in accordance with the embodiments disclosed herein an improved lock detector and a lock detection method.